I"m glad I came across this thread, I went mountain biking today for the first time in years... Actually, I went twice today, this morning with a buddy and then went back for round 2 myself later this afternoon. I've primarily had road bikes and was always fine clipping in there. More recently I went to a hybrid and have been using power grips. I really like them, I don't get into full roadie gear on my hybrid and can wear whatever sneakers I have on. I swapped them onto my MTB this morning and I have mixed feelings on them.. For the times I had to get out quickly, it was perfect, thats definitely the biggest benefit. The biggest drawback is flipping the pedal to get in quickly. Clipless definitely has the advantage there. There were a few times I just got frustrated and rode the pedal upside down but then when I wanted to grip it just wasn't there.
I may try platform pedals with pins - the VP's posted above seem good.. I guess its just the matter of learning how to use them properly to benefit from them.